Three-dimensional image of temporomandibular joint that was reconstructed from spiral CT data may have much diagnostic information. Several researches reported about the combination of 3-D TMJ image and six-degrees-of-freedom jaw movement data for the evaluation of jaw function. Very few studies have reported about accuracy of these 3-D images. The purpose of this research is to investigate the optimal scanning condition of spiral CT image and the distortion of reconstructed 3-D images3-D TMJ image of dry human skull was taken with spiral CT (Somatom Plus 4, Siemens Co). Same material was also scanned with 3-D digitizer (Microcode FN503, Mitsutoyo Co) to evaluate actual size of the imaging sample. Comparison of these two images was done with computer graphics. With the finest scanning condition, maximum error of the CT image was less than 2% and this accuracy was sufficient enough to evaluate slight inter joint space of TMJDistortion of CT image was mostly affected by table feed speed. Cut level CT number was very influential for size of CT image. When cortex bone CT number was applied as the cut level, size of the 3-D TMD image became 10 % smaller than that of actual TMJIt would be concluded that the standardization of scanning condition should be concerned for the evaluation of 3-D TMD image using CT dataPart of this research was presented at the 81 st General Session and Exhibition of the International Association for Dental Research, Goteborg, Sweden, June 25-28, 2003